Fuel cells are devices that directly convert the chemical energy of a fuel into electric energy through electrochemical oxidization of the fuel, such as hydrogen or methanol, in the cells and provide the electric energy. Thus, they draw attention as clean electric energy sources. In particular, polymer electrolyte fuel cells work at a lower temperature than other fuel cells, and thus are expected to be used in alternative power sources for automobiles, cogeneration systems for individual family homes, portable generators, and other applications.
Such polymer electrolyte fuel cells are each provided with at least a membrane electrode assembly including an electrolyte membrane and gas diffusion electrodes, each of which is composed of a laminate of an electrode catalyst layer and a gas diffusion layer, bonded to both surfaces of the electrolyte membrane. The electrolyte membrane mentioned here is a material having a strong acid group such as a sulfonic acid group or a carboxylic acid group in the polymer chain and having an ability to selectively allow protons to pass therethrough.
Polymer electrolyte membranes are used in fuel cells in environments where the amount of water varies. However, the polymer electrolyte membranes swell with water to show a great dimensional change, and thus they need improvement in durability and reliability. In order to reduce such a dimensional change, various ways of embedding a reinforcing membrane in the electrolyte are proposed.
For example, Patent Literature 1 discloses a composite membrane for polymer electrolyte fuel cells, comprising: (a) a stretched, expanded polytetrafluoroethylene membrane having a first main surface and a second main surface, having a microstructure of polymer fibrils and having a thickness of 0.06 mil (1.5 μm) to 0.8 mil (20 μm) and an average pore size of 0.05 to 0.4 μm; and (b) an ion exchange material impregnated throughout the microstructure of the membrane, the impregnated stretched, expanded polytetrafluoroethylene having a Gurley number of greater than 10,000 seconds, wherein the ion exchange material substantially impregnates the membrane so as to render the first main surface, the second main surface, and the whole of an interior volume of the membrane substantially uniformly occlusive.
Patent Literature 2 discloses an electrochemical cell membrane comprised of a composite membrane comprised of expanded polytetrafluoroethylene and ion exchange polymer as matrix polymer, the expanded polytetrafluoroethylene being made from polytetrafluoroethylene fine powder having a standard specific gravity (SSG) of no more than about 2.16, a breaking strength of at least about 5.5 lb force (24.5 N), and a stress-relaxation time of at least about 500 sec.
Patent Literature 3 discloses a composite membrane comprising (a) an expanded polytetrafluoroethylene membrane having an internal microstructure consisting essentially of nodes interconnected by fibrils, the nodes aligned substantially in parallel, being highly elongated and having an aspect ratio of 25:1 or greater; and (b) an ion exchange material impregnated throughout the membrane, the impregnated expanded polytetrafluoroethylene membrane having a Gurley number of greater than 10,000 seconds, wherein the ion exchange material substantially impregnates the membrane so as to render an interior volume of the membrane substantially occlusive.
Patent Literature 4 discloses a composite comprising a porous polymeric membrane, wherein the porosity of the membrane is at least partially filled with resin, the resin having a room temperature flexural modulus of greater than about 1 GPa, and wherein the membrane satisfies the following equation: 75 MPa<(longitudinal membrane tensile modulus+transverse membrane tensile modulus)/2. Patent Literature 4 includes no description about polymer electrolyte membranes.